Most vaccine side effects are mild and temporary, commonly including pain, swelling, or redness at the injection site, mild fever, chills, fatigue, and headache[7]. These reactions usually begin within the first few days after vaccination and resolve without specific treatment[5].
For COVID-19 vaccines, the most commonly reported side effects are headache, fever, fatigue, injection site pain, chills, muscle aches, and tiredness[2][3][6]. These typically appear within 24 hours and usually last one to three days. Injection site reactions have been reported in up to 90% of people, while fatigue and headache affect about 50–70% and 50% of recipients, respectively. Serious adverse effects, such as severe allergic reactions or hospital admissions, are rare (around 0.3–1%)[3][4][6].
The type and frequency of side effects can vary among different vaccines and between first and second doses. For example, mRNA vaccines (Pfizer-BioNTech and Moderna) are associated with more frequent mild to moderate side effects after the second dose[6]. Single-dose vaccines like Johnson & Johnson may result in similar side effect profiles but typically after their one dose[4].
Other common vaccines (such as those for influenza, measles-mumps-rubella, or chickenpox) also cause mostly mild reactions, such as localized pain or low fever, with severe reactions being extremely rare[5][1]. Certain vaccines, like the smallpox vaccine, carry a higher risk of adverse events, but these are still uncommon relative to the number of people vaccinated. For example, life-threatening reactions to the smallpox vaccine occur in 14–52 out of every million people vaccinated, and death occurs in 1–2 per million—primarily among those with weakened immune systems[1].
If any side effect is unexpected, persistent, or severe, medical advice should be sought. Most countries have systems for reporting adverse events following immunization, and ongoing monitoring helps ensure vaccine safety[5].
References
- [1] Vaccine Side Effects and Adverse Events – HistoryOfVaccines.org
- [2] Adverse Events of COVID-19 Vaccines in the United States
- [3] Immunization Reactions – Seattle Children’s Hospital
- [4] Exploring the reported adverse effects of COVID-19 …
- [5] Table. Common side effects following immunisation for vaccines …
- [6] How frequent are acute reactions to COVID-19 vaccination …
- [7] Vaccine Side Effects – HHS.gov
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